Conventionally, a thermal type flow meter is known, which measures a flow rate of fluid such as air or gas. The thermal type flow meter includes a heating element heated to a base temperature higher than a temperature of the fluid by a predetermined degree, an upstream temperature sensitive resistive element arranged upstream of the heating element in a flow direction of the fluid, and downstream temperature sensitive resistive element arranged downstream of the heating element in the fluid flow direction. The thermal type flow meter measures a flow rate of the fluid based on a detection temperature difference between the upstream and downstream temperature sensitive resistive elements, and further includes a bridge circuit which controls a temperature of the heating element to be the base temperature. For example, in JP06-160142A (corresponding to U.S. Pat. No. 5,369,994), as shown in FIG. 10, two bridge arms are connected in parallel between a power feed terminal 100 and a ground terminal 110. One of the bridge arms includes a temperature sensitive resistive element 130 which detects a temperature of a heating element 120, and an adjustable resistive element 140 resistance value of which is adjustable. The temperature sensitive resistive element 130 and the adjustable resistive element 140 are connected in series. The other one of the bridge arms includes a temperature sensitive resistive element 150 which detects a temperature of fluid, an adjustable resistive element 160 resistance value of which is adjustable, and a fixed resistive element 170 resistant value of which is fixed are connected. The temperature sensitive resistive element 150, the adjustable resistive element 160 and the fixed resistive element 170 are connected in series. A fixed voltage is applied between the power feed terminal 100 and the ground terminal 110. The adjustable resistive elements 140, 160 are made from thick films on a support, and resistance values of the adjustable resistive elements 140, 160 are adjusted such that a detection temperature difference between the temperature sensitive resistive elements 130, 150 becomes approximately constant.
In JP3817497 (corresponding to US2004/0069061), as shown in FIG. 11, two bridge arms of a bridge circuit are connected in parallel between a power feed terminal 200 and a ground terminal 210. One of the bridge arms includes a temperature sensitive resistor 230, which detects a temperature of heating element 220, and another temperature sensitive resistor 240, and the resistors 230, 240 are connected in series. The other one of the bridge arms includes a temperature sensitive resistor 250, which detects a temperature of fluid, and another temperature sensitive resistor 260, and the resistors 250, 260 are connected in series. A fixed voltage is applied between the power feed terminal 200 and the ground terminal 210. The four temperature sensitive resistors 230, 240, 250, 260, which constitute the bridge circuit, are made of the same material by the same process, and have almost same temperature coefficients.
In JP06-160142A, the detection temperature difference between the temperature sensitive resistors 130, 150 is adjustable by controlling resistance values of the resistors 140, 160 individually. However, because the adjustable resistors 140, 160 occupy a relatively large area on a circuit, the circuit may become large. Moreover, thick film resistors are generally trimmed by using laser or the like, so that an expensive equipment for laser trimming is required Additionally, thick film resistors are required to be protected with a silicone gel or the like and then to be covered with a cover after trimming. Thus, characteristics of thick film resistors may be changed in the protection and cover processes after trimming. As a result, a production process of thick film resistors may be complicated, and production equipments of thick film resistors may be expensive.
In JP3817497, the four temperature sensitive resistors 230, 240, 250, 260, which constitute the bridge circuit, are made of the same material by the same process, and have almost same temperature coefficients. Thus, a variation in a detection temperature difference between one another is relatively small without controlling resistance values of the resistors 140, 160 individually as in JP06-160142A. However, the four temperature sensitive resistors 230, 240, 250, 260 are actually different from one another in shape, temperature, and position on a sensor element. Hence, the detection temperature differences between one another are required to be adjusted individually in order to provide a high accuracy measurement of flow rate. The conventional technology described in JP3817497 does not have an adjusting function and is not capable of measuring a flow rate with high accuracy.